Published German Patent Application DE-OS No. 16 98 549 discloses a measuring apparatus of this type wherein a sensor for the heat conductivity and a sensor for the reaction heat of the gas components to be detected are arranged in the bridge branches of a measuring bridge. At low concentrations of the combustible gas, the reaction-heat effect exceeds the heat-conductivity effect, so that with increasing concentration of the combustible gas, also an increasing measuring signal can be tapped in the diagonal branch or measuring branch of the measuring bridge. With a further increase of the concentration of the combustible gas in air, the reaction-heat effect, because of the further decreasing oxygen concentration in the mixture, becomes smaller, and the effect of heat conductivity on the unbalancing of the measuring bridge increases. Depending on the balancing of the individual bridge branches with respect to one another, the measuring apparatus will be switched to full amplitude in the measuring branch starting at a predetermined heat-conductivity signal and thereby showing that the concentration of combustible gas mixed with air is above a predetermined limit. Thus, to that extent, a clear indication is obtained; otherwise, a decline of the reaction-heat signal with a further increasing concentration of the combustible gas would falsely indicate harmlessly low concentrations to the user of such known apparatus.
However, with the known measuring apparatus, it is not possible to measure the true content of the combustible gases mixed with air when a predetermined limit value for the reaction-heat measurement is exceeded.
German Published Patent Application DE-OS No. 31 27 431 discloses a monitoring instrument for the concentrations of combustible gases which considers the foregoing and suggests connecting a first bridge circuit for measuring the reaction heat in series with a second bridge circuit having its own compensation resistance which is designed exclusively for the utilization of the heat conductivity.
Doubling the number of measuring bridges is, however, expensive and doubles the complexity of the circuit and increases energy consumption.
A katharometer is disclosed in German Published Patent Application DE-OS No. 16 48 857 for detecting the level of combustible gases by measuring heat conductivity. This katharometer includes a measuring bridge in which a first sensor branch utilizes a heating filament as a heat-conductivity sensor exposed to the gas to be measured and a second sensor branch utilizes an encapsulated heating filament of the same kind for compensating for the effect of the changing ambient temperature. The indicating instrument is in the diagonal of the bridge. A disadvantage here is the high power requirement of the compensating element maintained at elevated temperature in the same way as the actual measuring sensor.